This invention pertains to the art of continuous casting and more particularly to continuous casting of tubes and round shaped rods, although it is also applicable to the casting of other shapes.
The invention is particularly applicable to a method and apparatus for effecting a fine grain structure in continuous cast copper alloy rods and tubes of various cross-sections and will be described with particular reference thereto. Those skilled in the art, however, will appreciate that the invention has broader applications and may be adapted for use with other alloys or materials in other environments.
In continuous casting of rods and tubes, one type of general casting system employed utilizes a stationary die wherein the casting is intermittently moved generally longitudinally in order to effect the required casting conditions. During a so-called withdrawal stroke, the casting moves fast enough so that only liquid metal enters the cooled length of the die for causing intimate die metal contact. This stroke is followed by a dwell period during which the casting is stopped or slowed down so that it will exit from the solidification zone at the proper temperature.
Heretofore, in continuous casting of copper and copper alloy rods and tubes using the above-described as well as other techniques, gross directional solidification occurred in the rods during alloy transition from the liquid to the solid state. Such gross directional solidification resulted in the development of crystals or grains which grew generally opposite to the direction of heat flow. The grains were usually quite long in the direction of casting and coarse to fine at right angles to that direction. As a result, cold drawing or working of continuous cast copper alloy rods had previously been very difficult or impossible due to the thick grain boundaries generally associated with these structures. Attempts at such cold drawing had undesirably caused cracks and imperfections to appear in the rods as a result of the coarse grains. This, in turn, generated scrap and/or unacceptable end products.
Moreover, if the liquid metal is too hot and/or the casting speed too slow, the grain structure takes on a coarse elongated configuration generally in the direction of casting. Such grain structure is wholly undesirable for metals which are to be subsequently cold drawn. Thus, while it is possible to vary the casting parameters within normal casting practice so as to alter the grain structure of the cast rod, it had not been possible to alter these parameters by an amount, or to the degree, necessary to effect a grain structure which was readily conducive to cold working.
It has been known to be beneficial to all casting and metal working schemes to have the grain boundaries be as thin as possible. For this reason, it was considered desirable to develop an arrangement which would readily facilitate obtaining such fine grain structures in continuous cast copper alloy rods and tubes. Such rods and tubes would then satisfactorily accommodate subsequent cold drawing or working. Thus, U.S. Pat. No. 4,315,538 disclosed a method and apparatus to effect a fine grain size in continuous cast metals. This involved the use of a continuous casting die totally submerged in a reservoir of liquid alloy material and the use of feed openings in the die arranged so that the liquid metal entering the die would impart a generally cyclonic motion at the interface zone between the liquid and solid alloy material. This cyclonic motion caused shearing of primary dendrites in the alloy material from adjacent the internal side wall of the die and distributed those dendrites across the interface zone to provide nuclei for equiaxed crystals, thereby preventing the formation of thermal gradients in the alloy material of a sufficient magnitude to produce gross directional solidification at the interface zone.
Using the continuous casting arrangements disclosed in U.S. Pat. No. 4,315,538 has taught that their ability to produce fine grain structures in tubes with wall thicknesses of more than 0.5 inch could be improved. Accordingly, the subject of this invention is a die construction for use with the same type of continuous casting apparatus, but with an improved ability to produce a fine grain structure in tubes with wall thicknesses more than 0.5 inch, as well as in other cast shapes, such as round shaped rods.